This invention relates to production of single-cell protein from potato processing waste.
Production of yeast as a primary food source may provide a partial solution to the world food problem. Yeasts have been recognized as dependable food sources for many years. They are very good sources of protein, being comparable to meat and soybean. They also supply amino acids in quantities that make them as good a source as soybean, milk, egg and meat. In addition, they are valuable sources of the B-vitamins. Yeasts are therefore prime candidates to help eradicate protein deficiencies and the vitamin deficiency diseases such as beriberi and pellagra.
Saccharomyces cerevisiae and Candida utilis have been commonly used as food yeasts. As to gross composition, Saccharomyces cerevisiae grown on molasses contains 52.4% protein, 37.1% carbohydrates, 1.7% fat, and 8.8% ash. Its elemental composition is 45% carbon, 32% oxygen, 9% nitrogen, 6% hydrogen, and 8% inorganic matter. Candida utilis grown on sulfite liquor contains 5.8% moisture, 8% nitrogen, of which nucleic acid nitrogen is 0.4%, 29.6% polysaccharide, 4.8% fat, 0.8% fiber, and 9.0% ash. Thiamine is present as 5 .mu.g/g dried cell material, riboflavin as 45 .mu.g/g, niacin as 415 .mu.g/g, and pyridoxine as 29 .mu.g/g. Using the common factor total N .times. 6.25, about one-half of the dry weight of yeast cells is crude protein. The content of carbohydrates, fats, and proteins in yeasts varies according to growth conditions and substrate employed. Generally, yeasts are used to supplement cereals, to flavor foods, and to add a nutritional supplement to prepared foods.
Food yeasts are grown for human consumption, while fodder yeasts are grown for animal consumption. Usually food yeasts are non-fermenting stable whole yeast cells, carefully prepared and dehydrated to yield flakes or powders intended for improving the human diet. Food yeasts differ from fodder yeasts mainly in quality characteristics. Food yeasts must exhibit lower bacterial and mold contents, higher levels of vitamins, higher protein content, bland flavor, light color, and absence of pathogenic microorganisms. They also must not contain added ingredients or fillers.
Today cultivation of microorganisms for food, since 1966 called single-cell protein (SCP), has become a fairly vigorous field. Yeasts and bacteria have been grown on hydrocarbons, cellulose accumulation, and even on livestock wastes. More traditional methods include use of molasses, sulfite liquor, and whey as growth media.
According to the World Survey of Fermentation Industries, 1963, dried yeast is produced in twenty countries, with a total annual production estimated at 180,000 metric tons dry weight. Of this total, 60% is produced in eastern Europe and the U.S.S.R. as fodder yeast, with molasses and spent sulfite liquor as principal carbon sources. North America produces approximately 37,000 metric tons of dried yeast annually.
Some work has been done using potatoes and potato processing waste water as growth media for various species of yeast. The yeast is grown principally as fodder yeast for feeding cattle and pigs. Since potatoes have a protein-carbohydrate ratio of 1:10 or 1:12, they tend to cause over-fattening of meat carcasses with a resultant loss of muscle mass if they are unfortified when fed to meat animals. The gross composition of potatoes is water 77.9%, protein 2.0%, fat 0.1%, ash 1.0%, and total carbohydrate 19.1%. Fiber makes up 0.4% of the total mass of a potato, with free sugars making up 0.8% and starch making up 14.7%. Vitamins contained in 100 g of raw potato are niacin 1.5 mg, thiamine 100 .mu.g, riboflavin 40 .mu.g, vitamin A a trace, and vitamin C 17 mg. The quantity of essential amino acids averages around 100 mg of each per 100 g of raw potato.
Working in Poland, Janicki et al. found that they achieved best yeast growth using 18-20% w/v mixtures of dried, saccharified potato in water. They added nitrogen and phosphate salts and grew Candida utilis in aerated tanks at 30.degree.-32.degree. C. Continuous growth with temperature and pH control gave best results, with a yield in the range of 450-530 g of yeast cells per kg of sugar present in the potato medium.
Researchers in Sweden were able to cultivate Candida utilis on waste water from potato processing plants. The waste water contained 1.5-4.0% solids, primarily potato starch, and was sterilized before use. Endomycopsis fibuliger, a yeast that produces external amylases, was used to break down the starch in the waste water into glucose, which is utilized more rapidly by C. utilis. The yeast yield was 45 mg of cells per 100 mg of starch supplied in the waste water. See, for example, Food Engineering, Nov. 1976, pp. 66-67.
Reiser, at the University of Idaho, successfully cultivated Candida utilis on the waste water from potato starch plants in 1954. The waste water contained 1.2% solids, of which 37% was protein, 12% was sugar, and 0.8% was starch. It was not sterilized before use and no additional growth factors were needed. The yeast was grown in a 30 liter laboratory fermentor at 30.degree.-32.degree. C., at a pH of 5, which is not the optimal pH for the yeast, but which does reduce growth of bacterial contaminants. The yeast yield was about 40% of the solids supplied in the waste water.
The present invention is based on a recognition that very dense potato waste filter cake from potato processing plants can be used as a substrate for the growth of Candida utilis. Currently this untreated filter cake is being fed to cattle. The growth of yeast on the filter cake can increase protein content to an extent that it would be a more complete diet for animals.